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DC System Riddle No.2 - Number of cell of battery
For selection the number of cell of battery in 220vdc system there is two ways one way consider 108 cells another way consider 110 cells; which one is preferred?
Author : Madjidi - From: Iran
 
#1
Tue, June 15th, 2010 - 15:17
It is related to acceptable minimum and maximum system voltage, end-voltage of each cell which specified by manufacturer regarding rated capacity (Ah) of cells (e.g. C8=200 AH, in 8 hour continues discharge time) and permissible maximum cell voltage.
The standard end-of-discharge voltage used for lead-acid cells is 1.75 volts per cell (Vpc) in North America. In other parts of the world, 1.80 or 1.81 Vpc is frequently used.
These end voltages typically apply to cells rated at discharge rates equal to or greater than 1 h. The use of these end voltages ensures that the potential for damaging a lead-acid battery due to overdischarge (i.e., caused by overexpansion of the cell’s plates) is minimized.
For discharge rates less than 1 h, a lower end voltage may be used without causing any damage to the cell’s plates. For example, high-performance lead-acid cells rated at the 15-min rate may use 1.67, 1.64, or 1.60 Vpc as an end voltage; however, 1.67 Vpc is becoming the industry standard.
In lead acid battery, the maximum cell voltage is 2.33 V (maximum voltage of float charging) and assume 1.75 V is typical as end-voltage cell in our country, so if we have 184V and 260 V as acceptable minimum and maximum DC system voltage respectively, both of them (108 and 110 number cells) can be acceptable.

1.75*108=189>184 V , 108*2.33=251.6<260 V
1.75*110=192.5>184 V , 110*2.33=256.3<260 V

Since the standard ratings for individual cells include an end voltage, the requirements of a dc system often dictate the use of other than the standard end voltage. The voltage window (i.e., the voltages from the minimum to maximum system voltage) for applications varies. Telecommunications applications often have a narrow voltage window due to the limitation of acceptable equipment voltages; however, this is changing with new equipment that can tolerate lower input voltages being introduced in the market. Electric utility and industrial control-voltage windows generally use the standard cell end voltage but are sometimes limited by the minimum system-voltage limit. UPS applications can offer the largest voltage window, since the inverter input can be designed to accept a wide range of voltage without affecting the inverter’s output voltage.
End voltage is important in battery sizing because the lower the end voltage, the more energy that can be removed from the battery. This results in a smaller battery for the application. Conversely, higher end voltages increase the size of the battery needed to perform the desired design function.
When determining end voltage at the battery terminals for the purpose of sizing the cell, do not forget to factor in the voltage drop to the loads. This may require an analysis of each load to determine which represents the worst case.
 
Author : Hamid - From: Iran- Firouzabad in Fars
 
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